Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold magnetization and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell changes due to the orientation of the magnetization of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0.” One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.
FIG. 1A illustrates a magnetic tunnel junction (“MTJ”) 130 for a conventional MRAM device. The MTJ 130 includes reference layer 132, which is a magnetic layer, a non-magnetic tunneling barrier layer 134, which generally is an insulator at large layer thickness but at small layer thickness admits the flow of an appreciable quantum mechanical tunnel current, and a free layer 136, which is also a magnetic layer. The magnetization direction of the magnetic layers of the MTJ 130 can either be in the planes of those layers or perpendicular to the planes of those layers. As shown in FIG. 1, the magnetic reference layer 132 has a magnetization direction perpendicular to its plane. Also as seen in FIG. 1, the free layer 136 also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees. Because the magnetization direction of the magnetic layers of the MTJ 130 is perpendicular to the planes of those layers, the MTJ 130 shown in FIG. 1A is known as a perpendicular MTJ.
As shown in FIG. 1A, electrical contact layers 170, 172 can optionally be used to provide electrical contact to the MTJ 130. When the magnetization of the free layer 136 is oriented in a direction parallel to the magnetization direction of the reference layer 132, electrons will be more likely to tunnel across the tunneling barrier layer 134, and thus resistance across the MTJ 130 will be lower. Alternatively, when the magnetization of the free layer 136 is oriented in a direction that is anti-parallel to the magnetization direction of the reference layer 132, electrons will be less likely to tunnel across the tunneling barrier layer 134, making the resistance across the MTJ 130 significantly higher. It is these different resistances that can be used to distinguish and store a digital “1” or “0” bit.
The MTJ 130 may also form part of a larger MTJ stack 100, as shown in FIG. 1B, which may include a number of other optional layers that can be used to facilitate operation of the MTJ. As described in connection with FIG. 1A, the MTJ stack 100 of FIG. 1B may include electrical contact layers 170, 172 for providing electrical contact across the MTJ stack 100, including the MTJ 130. The MTJ 130 may be disposed above an antiferromagnetic layer or a synthetic antiferromagnetic (“SAF”) structure 120, which may include multiple layers as shown in FIG. 1B. For example, as shown in FIG. 1B, the SAF structure 120 may include two or more thin magnetic layers, including a lower “SAF1” layer 122 and an upper “SAF2” layer 126 having opposite or anti-parallel magnetization directions separated by an antiferromagnetic coupling layer 124 or spacer layer that is not magnetic. The SAF structure 120 also may be formed over a seed layer 110, as shown in FIG. 1B, and over a substrate (not shown). Note that as used herein, terms such as “lower,” “upper,” “top,” “bottom,” and the like are provided for convenience in explaining the various embodiments, and are not limiting in any way.
Spin transfer torque or spin transfer switching, may be used in connection with an MTJ 130. In such a configuration, a filter layer 150 may be used to alter the spin of electrons passing through the MTJ 130. For example, the filter layer may be a polarizer layer designed to further align the spin of electrons (i.e., to further “polarize” the electrons) passing through the MTJ 130 beyond the alignment already provided by the reference layer 132. U.S. patent application Ser. No. 14/814,036, filed by Pinarbasi et al., and assigned to the assignee of this patent document describe using a polarizer layer. The disclosure of U.S. patent application Ser. No. 14/814,036 is incorporated herein by reference in its entirety. The spin-aligned or “polarized” electrons are used to change the magnetization orientation of the free layer 136 in the MTJ 130. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer, like the filter layer 150 or the reference layer 132, polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer, thus producing a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer 136 of the MTJ 130, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, which can be used to write either a “1” or a “0” based on whether the free layer 136 is in the parallel or anti-parallel states relative to the reference layer.
As shown in FIG. 1B, the filter layer 150 and a nonmagnetic filter coupling layer 140 or “spacer” are disposed above the free layer 136 of the MTJ 130. The filter layer 150 is a magnetic layer that has a magnetic direction in its plane, but is perpendicular to the magnetic direction of the reference layer 132 and free layer 136. In addition to the polarizing effects from the reference layer 132, a polarizer layer can be used as the filter layer 150 to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure 100 in the manner described above.
As shown in FIG. 1C, the MTJ stack 100 can also use a precessional spin current magnetic layer instead as a filter layer 150. The precessional spin current magnetic layer is described in detail in U.S. patent application Ser. No. 14/814,036, filed by Pinarbasi et al., and assigned to the assignee of this patent document. In such an MRAM device, a precessional spin current magnetic layer is physically separated from the free magnetic layer of a magnetic tunnel junction and is coupled to the free magnetic layer by a non-magnetic spacer 140. Additionally, a precessional spin current structure for MRAM is described in U.S. patent application Ser. Nos. 15/445,260 and 15/445,362, both of which are filed by Pinbarsi et al. and are assigned to the assignee of this document. The disclosure of U.S. patent application Ser. Nos. 15/445,260 and 15/445,362 are both incorporated herein by reference in their entirety.
The magnetic layers of the MTJ stack 100 shown in FIG. 1, such as the reference layer 132 and the magnetic layers 122, 126 of the SAF structure 120, generally emit stray magnetic fields as part of their normal operation. The stray fields emitted by these magnetic layers can interfere with and degrade the performance of other layers of the MTJ stack 100. For example, stray fields from magnetic layers of the MTJ stack 100 can impinge on and degrade the performance of the free layer 136, because they cause an asymmetry in the switching of the magnetization direction of the free layer 136. In other words, these stray magnetic fields can make it easier to align the magnetization direction of the free layer in one direction than in the other direction. This can result in the need for a larger current to switch the magnetization direction of the free layer 136 in the more difficult direction. Such an asymmetry can also be characterized as an offset of the center of an M-H hysteresis loop of the MTJ 130 (i.e., the hysteresis observed when measuring the magnetization or “M” and the magnetic field “H” of the MTJ using a magnetometer). The asymmetry of the switching current in such situations can result in added costs and increased design requirements for the corresponding electronic circuitry. It also can be detrimental to the memory retention time, because of the reduced energy barrier against thermal reversal in one of the two storage states. Additionally, the asymmetry of switching current generally can be deleterious to the error rate performance of the MTJ 130 and any memory device in which it is used.
The stray magnetic fields of the magnetic layers of the MTJ stack 100, such as the reference layer 132 and the magnetic layers 122, 126 of the SAF structure 120, can also impinge on and degrade the performance of a filter layer 150, such as a polarizer layer or a precessional spin current magnetic layer, when such a layer is used. In particular, the stray magnetic fields can negatively affect the performance of a precessional spin current magnetic layer used as a filter layer 150 because they introduce an asymmetry in the dynamic magnetic rotation of that layer. This asymmetry results in a performance degradation and potential increased costs in the device.
The negative effects of the stray magnetic fields are more pronounced as the magnetic layers emitting those fields become closer to the layers that they are affecting. In addition, it can be expected that this effect will become increasingly pronounced as the lateral size (e.g., the diameter) of the MTJ 130 decreases. Also, as layers of the SAF structure 120 or the reference layer 132 get closer to the free layer 136 or the filter layer 150, the stray magnetic fields have more of an impact on the free layer 136 and the filter layer 150. As a consequence, it is desirable to reduce the negative effects of these stray fields.
Prior approaches to reducing stray magnetic fields have included changing the magnetic moment of one or more layers of the SAF structure 120 and the reference layer 132 by changing the magnetic volume of those layers. But this can cause undesirable effects. For example, increasing the magnetic volume of the reference layer 132 can reduce that layer's perpendicular anisotropy, which leads to degraded performance of the MTJ 130 device. Additionally, increasing the magnetic volume of the reference layer 132 can also reduce the pinning of the reference layer 132 to one or more of the layers of the SAF structure 120, which could cause the MTJ 130 device to perform poorly and can reduce the stability of the reference layer 132.